1. Field of the Invention
The present invention relates to a process for separating off lower alkanols from other organic compounds from the group comprising other alcohols, polyalcohols, ethers, oxo compounds, esters of carboxylic acids and of carbonic acid, halogenoaliphatics, amines, amides, hydrocarbons, carboxylic acids, nitriles or mixtures of a plurality thereof. The other organic compounds have in all cases at least one carbon atom more than the alkanol to be separated off, where in the case of halogenoaliphatics, halogen substituents are counted as further carbon atoms. The separation is carried out by pervaporation or vapour permeation on a hydrophilic membrane. For this purpose, according to the invention, a water content from 1 to 30% by weight, preferably from 5 to 15% by weight, based on the amount of mixture and water, is maintained or set and maintained.
2. Description of the Related Art
The process of the invention makes simplified separations possible, particularly in those cases where distillative separation processes fail because of the presence of an azeotrope. Even with other separation problems, the process of the invention gives substantial advantages in energy consumption. Important industrial problems of this type are, for example, the separation of methanol from i-propanol, dimethyl carbonate (DMC), methyl tert-butyl ether (MTBE) and tert-amyl methyl ether (TAME). The separation of such mixtures has hitherto been carried out, for example in the case of DMC, by two-pressure distillation (DE-A 26 07 003), since it is known that the composition of azeotropic mixtures is dependent on pressure. The degree of separation obtained by this method is often insufficient and has to be supplemented by other physical processes, for example by crystallization. In addition, pressure apparatus always result in higher capital costs; because the temperature level is increased by the pressure, increased formation of byproducts is also always to be expected. In addition to distillation under increased pressure, attempts have also been made to separate mixtures of the type described, by extractive distillation. In the case of the separation of the methanol/DMC azeotrope, water is preferably used as the extraction agent (DE-A 24 50 856). The ratio of water to dimethyl carbonate required is here 20:1. This large amount of water has to be separated from the methanol again in a further distillation column. It is an additional disadvantage here that water has more than 4 times the heat of vaporization and more than twice the thermal capacity of organic compounds; both lead to increased energy consumption. Even improvement of the extractive distillation by use of organic solvents in place of water still suffers from the disadvantage of these additives having to be worked up and recycled (EP-A 1780; DE-A 27 06 684 and DE-A 27 37 265).
There have therefore already been attempts to use membrane technology to separate mixtures which are difficult to separate. EP-A 331 846 thus describes the separation of short-chain alcohols from oxygen-containing organic compounds, such as ethers, aldehydes, ketones or esters, with the aid of a multilayer membrane. The separating membrane in this arrangement comprises either a polyvinyl alcohol membrane crosslinked with aliphatic polyaldehydes, or a resin which is also used in ion exchangers and contains acid groups which have been modified by quaternary ammonium salts. A woven polyester fabric is used as the support material for the multilayer membrane, a porous support membrane which is furthermore used comprises a woven polysulphone fabric. This membrane is a relatively complicated structure. As a result of its chemical composition, the operating temperature is limited to 70.degree. C. and thus gives a low maximum flux of 1.5 kg/m.sup.2 h. This imposes limits on industrial use. The described permeate enrichment of methanol from 73% to nearly 93% in the case of separation of methanol/DMC means that the methanol isolated by this method has to be freed from the residual 7% of DMC in a further operation.
According to the description in EP-A 423 949, an attempt is made to overcome the disadvantages described for the process of EP-A 331 846 by means of another membrane comprising a blend of polyvinyl alcohol and polyacrylic acid on polyacrylonitrile as support membrane. In the case of the separation of methanol/DMC, enrichment from 73% of methanol to about 95% of methanol is achieved at a flux of about 2 kg/m.sup.2 h; this enrichment represents, on the retentate side, the minimum requirement for purified DMC for use in further processes. A membrane obtained by plasma polymerization, which is preferably a composite membrane, is, according to the description of EP-A 476 370, used for the separation of water of reaction from an esterification mixture. An esterification mixture according to this description comprises unreacted carboxylic acid, unreacted alcohol, the ester desired as the reaction product, the water of reaction formed during the reaction and an acid esterification catalyst. The separation of the water of reaction on the membrane is then followed by a separation of the retentate into the ester product stream and into the acids and alcohols which have not yet reacted and are to be recycled. The process of EP-A 476 370 is accordingly characterized in that, apart from the water, no substantial organic constituents pass through the membrane; it is particularly important here that the esterifying alcohol does not pass through the membrane.